1. Field of the Invention
The present invention relates to ultrasonic sensors and, in particular, an ultrasonic sensor that includes a piezoelectric element and an input/output terminal electrically coupled thereto and that can be used in automotive corner sonar or back sonar, for example.
2. Description of the Related Art
An ultrasonic sensor uses ultrasonic waves in sensing and detects an object by intermittently transmitting an ultrasonic pulse signal and receiving a reflected wave from the obstacle present in neighboring areas. An ultrasonic sensor can be employed in automotive back sonar, corner sonar and, additionally, a parking sensor for detecting the presence of a space to an obstacle, such as a side wall, in parallel parking.
An example of this type of ultrasonic sensor is described in Japanese Unexamined Patent Application Publication No. 2000-32594. FIG. 1 is a cross-sectional view of an ultrasonic sensor 30 illustrated in this patent literature. The ultrasonic sensor 30 includes a case 31 including a bottom portion 32 and a side wall portion 34, a piezoelectric element 35, a sound absorber 36, an insulation material 37, and a cable 40. The piezoelectric element 35 is fixed to the inner surface of the bottom portion 32 of the case 31 and has a first electrode electrically coupled to the case 31. The inside of the case 31 is filled with the sound absorber 36 and the insulation material 37 having elasticity. A temperature-compensating single-panel capacitor 38 is embedded in the insulation material 37. The single-panel capacitor 38 has a first external electrode connected to the case 31 and a second external electrode connected to a second electrode of the piezoelectric element 35 with a lead 39 disposed therebetween. The cable 40 includes two signal lines 41 for use in inputting and outputting a signal. The two signal lines 41 are connected to their respective external electrodes of the single-panel capacitor 38.
A traditional ultrasonic sensor illustrated in FIG. 1 achieves good reverberation characteristics by being filled with the insulation material 37 having elasticity. However, such an ultrasonic sensor having a pin terminal structure in which a pin protrudes from a case has two major drawbacks described below.
(1) To suppress vibration of the side wall of the case and obtain good reverberation characteristics, it is necessary to fill the inside with an insulation material having a high modulus of elasticity for efficiently suppressing vibration of the case (hereinafter referred to as “filler”). However, if the inside is filled with a filler having a high modulus of elasticity, not all vibration transmitted from the side wall of the case toward the filler can be absorbed by the filler, and the vibration is transmitted to the pin terminal. This vibration leaks through the pin terminal to a substrate on which the sensor is implemented. The leakage of the vibration through the terminal is hereinafter referred to simply as “vibration leakage.” If there is vibration leakage, an unnecessary signal component (pseudo noise) is detected, and this is a serious problem for an ultrasonic sensor for sensing an object.
(2) In contrast to the above situation, in order to have a structure that prevents transmission of vibration to the pin terminal and avoids vibration leakage, it is necessary to fill the inside with a filler having a low modulus of elasticity. However, if the inside is filled with such a filler having a low modulus of elasticity, vibration of the side wall of the case cannot be sufficiently suppressed, and this increases the reverberation time. If the reverberation time is long, an obstacle at a short distance is not detectable.
FIG. 2 is a conceptual illustration of reverberation characteristics and vibration leakage characteristics with respect to a modulus of elasticity of a filler. In FIG. 2, the curve R represents the reverberation characteristics, and the curve V represents the vibration leakage characteristics. The horizontal axis indicates the modulus of elasticity, and the vertical axis indicates the time. The vibration leakage characteristics are a change in reverberation time between a discrete state of an ultrasonic sensor and a state where the ultrasonic sensor is implemented on a substrate. As illustrated, the reverberation time reduces with an increase in the modulus of elasticity of the filler, whereas the vibration leakage increases with an increase in the modulus of elasticity.
FIGS. 3A, 3B, and 3C illustrate vibration characteristics of three ultrasonic sensors having different moduli of elasticity. FIG. 3A illustrates characteristics of an ultrasonic sensor filled with elastic resin having a relatively low modulus of elasticity; FIG. 3C illustrates characteristics of an ultrasonic sensor filled with elastic resin having a relatively high modulus of elasticity; and FIG. 3B illustrates characteristics of an ultrasonic sensor filled with elastic resin having a modulus of elasticity between that illustrated in FIG. 3A and that in FIG. 3C. For the example of FIG. 3A, whose attenuation pattern is simple, no vibration leakage occurs, but the reverberation time is long. For the example of FIG. 3C, in which multiple types of vibration interfere with each other and thus a complex attenuation pattern appears, vibration leakage occurs. For the example of FIG. 3B, whose attenuation pattern is between that illustrated in FIG. 3A and that in FIG. 3C, vibration leakage occurs and reverberation time is long.
As described above, simply selecting an appropriate modulus of elasticity is insufficient for adequately improving both reverberation characteristics and vibration leakage.